Various means and devices have been utilized with electrochemical cells and particularly electrochemical batteries to prevent untoward occurrences, when the battery is abused or subject to adverse conditions such as short circuiting, voltage reversal and heat generation. Safety devices such as fuses and thermal switches interrupt current flow and battery operation by opening up a switch in the circuit. However, such devices permit the battery to continue operating upon replacement of a fuse or cooling of the battery, even though the condition which caused the original heat generation remains.
Devices, including transistor gating elements such as MOSFETs (metal-oxide semiconductor field-effect transistors) have been utilized, e.g., U.S. Pat. No. 5,179,337, as switches, to turn off current upon the occurrence of an untoward condition, such as at low voltage which may affect rechargability and possible result in cell reversal. General use of the MOSFET type of cutoff safety device is predicated on there being only two conditions of either on or off (i.e. the circuit being fully broken or fully closed). However, a factor not taken into account with the use of MOSFETs is the fact that the MOSFET does not immediately shut down, i.e. being either in the on or off state. Instead, as the referenced threshold gate to source voltage is reached, the MOSFET changes resistance from a low resistance device to a resistive device and eventually to a very high resistance or open device. During this transition stage, the MOSFET heats up under the high currents with resultant damage if the dissipated power exceeds the chip capabilities.
It is therefore an object of the present invention to provide a safety device which shuts down the discharge of the battery and keeps it in the latched off mode until the load is removed.
It is a further object of the present invention to provide the safety device with thermal protection and sensor switches outside of the discharge circuit but within the gate circuit.
It is yet another object of the present invention to provide low voltage protection with latching, high temperature protection with latching and short circuit protection with latching.
These and other objects, features and advantages of the present invention will become more evident from the following description and drawings in which: